In order to do this, you need to create a AdvancedDataSeriesFormat List variable on the screen. If you're using OS 10, you can prepare this variable on the onReady event of the screen. On this list, append a record with the series name and the following json code:

"{dashStyle: 'longdash'}"

Then, assuming you called your list variable CustomSeriesStyle, set the AdvancedFormat parameter on your chart to something like